This invention relates to antennas.
The invention is more particularly concerned with radar antennas, such as for ships.
Conventional marine radar antennas are of bar shape and are mounted horizontally to rotate about a vertical axis. A slotted waveguide extends horizontally across the width of the antenna, the slots opening along a side of the waveguide into a horn. In order to achieve a beam with a relatively narrow width in elevation, the aperture of the horn in a vertical direction has to be relatively large. This results in an antenna having a relatively large size in the vertical direction. This is a disadvantage because it increases the wind resistance of the antenna so that it must be made relatively robust, have bearings of a heavy construction and be driven by a high power motor.
It has long been known that the dimensions of a radar antenna can be reduced by using a dielectric material. The dielectric has the effect of constraining the microwave energy as it emerges from the antenna and can enable the use of a lower profile antenna shape (xe2x80x9cGain enhancement of microwave antennas by dielectric-filled radomesxe2x80x9d, James et al, Proc. IEE, vol 122, no 12, December 1975, pp 1353-1358). WO95/29518 describes an antenna with several plates of dielectric material extending parallel to the direction of the main energy beam.
It is an object of the present invention to provide an alternative antenna.
According to one aspect of the present invention there is provided an antenna including a waveguide extending along a first direction and arranged to propagate energy from a face of the guide in a second direction at right angles to the first direction, the antenna including a dielectric member of generally plate shape having an edge extending parallel to the face of the guide and having opposite surfaces facing in directions orthogonal to the first and second directions, and the dielectric member having at least one discontinuity on at least one of the surfaces arranged to scatter energy and enhance the properties of the energy radiated from the antenna.
The discontinuity preferably includes a step extending along the length of the dielectric member. The dielectric member may have two steps facing in opposite directions. The dielectric member may have a step on both surfaces and preferably has two steps facing in opposite directions on both surfaces. The or each discontinuity may be provided by a strip secured to each surface of the dielectric member to extend along its length. The antenna preferably has a single dielectric member, the thickness of the dielectric member being substantially less than the height of the antenna. The dielectric member is preferably of a foamed plastics material. The antenna preferably includes a polarisation grid located forwardly of the face of the waveguide, the antenna including two horn plates extending forwardly of the polarisation grid and a rear edge of the dielectric member being located between the horn plates. The or each discontinuity may be located forwardly of the horn plates. The location of the or each discontinuity is preferably selected to produce reflections that are substantially 180xc2x0 out of phase with extraneous energy produced within the antenna. The location of the or each discontinuity is preferably selected to control sidelobes of a beam of the energy and to enhance peak gain. The dielectric member may be supported by an expanded foam material, which may be contained within an outer radome that extends rearwardly along the waveguide.
According to another aspect of the present invention there is provided a marine radar antenna including a waveguide extending along a first, horizontal direction for rotation about a vertical axis and arranged to propagate energy forwardly in a second, horizontal direction from a face of the guide at right angles to the first direction, the antenna including a dielectric member of generally plate shape having an edge extending parallel to the face of the guide and having opposite surfaces facing vertically up and down, and the dielectric member having at least one discontinuity on at least one of the surfaces arranged to scatter energy, to control sidelobes of a beam of the energy and to enhance peak gain.
A radar antenna for a ship, according to the present invention, will now be described, by way of example, with reference to the accompanying drawings.